Phase Transition of Spindle-Associated Protein Regulate Spindle Apparatus Assembly

Spindle assembly required during mitosis depends on microtubule polymerization. We demonstrate that the evolutionarily conserved low-complexity protein, BuGZ, undergoes phase transition or coacervation to promote assembly of both spindles and their associated components. BuGZ forms temperature-dependent liquid droplets alone or on microtubules in physiological buffers. Coacervation in vitro or in spindle and spindle matrix depends on hydrophobic residues in BuGZ. BuGZ coacervation and its binding to microtubules and tubulin are required to promote assembly of spindle and spindle matrix in Xenopus egg extract and in mammalian cells. Since several previously identified spindle-associated components also contain low-complexity regions, we propose that coacervating proteins may be a hallmark of proteins that comprise a spindle matrix that functions to promote assembly of spindles by concentrating its building blocks. [Display omitted] •The spindle regulator BuGZ undergoes temperature-dependent phase transition•Phase transition of BuGZ relies on the aromatic and hydrophobic Phe and Tyr•BuGZ droplets promote microtubule polymerization•Phase transition of BuGZ promotes assembly of spindle and its matrix A protein associated with the mitotic spindle must undergo a phase transition to promote microtubule polymerization and spindle assembly, suggesting that the biophysical properties associated with liquid demixing may shape the characteristics of a hypothesized but elusive spindle matrix.